PEESPIRATION. 419 



The sweat poured out by the sudoriparous glands is a 

 transparent colorless liquid, with a peculiar odor, varying 

 in different races, and in the same individual in different 

 regions of the Body. Its quantity in twenty-four hours is 

 .subject to great variations, but usually lies between 700 and 

 2000 grams (10,850 and 31,000 grains). The amount is in- 

 fluenced mainly by the surrounding temperature, being 

 greater when this is high; but it is also increased by other 

 things tending to raise the temperature of the Body, as 

 muscular exercise. The sweat may or may not evaporate 

 .as fast as it is secreted ; in the former case it is known as 

 insensible, in the latter as sensible perspiration. By far 

 the most passes off in the insensible form, drops of sweat 

 only accumulating when the secretion is very profuse, or the 

 surrounding atmosphere so humid that it does not readily 

 take up more moisture. The perspiration is acid, and in 

 1000 parts contains 990 of water to 10 of solids. Among 

 the latter are found urea (1.5 in 1000), fatty acids, sodium 

 chloride, and other salts. In diseased conditions of the 

 kidneys the urea may be greatly increased, the skin supple- 

 menting to a certain extent deficiencies of those organs. 



The Nervous and Circulatory Factors in the Sweat 

 Secretion. It used to be believed that an increased flow of 

 Hood through the skin would suffice of itself to cause in- 

 creased perspiration; but against this view are the facts 

 that, in terror for example, there may be profuse sweating 

 with a cold pallid skin; and that in many febrile states the 

 skin may be hot and its vessels full of blood, and yet there 

 may be no sweating. 



decent experiments show that the secretory activity of 

 the sweat-glands is under the direct control of nerve-fibres, 

 and is only indirectly dependent on the blood-supply in 

 their neighborhood. Stimulating the sciatic nerve of the 

 freshly amputated leg of a cat will cause the balls of its 

 feet to sweat, although there is no blood flowing through 

 the limb. On the other hand, if the sciatic nerve be cut, 

 so as to paralyze it, in a living animal, the skin arteries di- 

 late and the foot gets more blood and becomes warmer; 

 but it does not sweat. The sweat-fibres originate in certain 



